The present invention relates to a method for the removal of an ester from a vapor mixture. The present invention also relates to an apparatus for carrying out this method, and the use of said apparatus in said method, as well as use of the method or apparatus in the production of a lactide or a polylactic acid polymer.
Removing an ester from a vapor mixture is of utility in the treatment of vapors resulting from vacuum ‘overhead’ systems used to remove volatile low molecular weight species (‘lows’) in polycondensation or ring-opening polymerization processes, such as in the production of polyesters prepolymers and resins. For example, the removal of cyclic diesters of an alpha-hydroxycarboxylic acid such as a lactide finds utility in the production of lactide and its polymers.
One method of production of lactide is by means of prepolymerization from lactic acid and subsequent thermal catalytic depolymerization at low pressures. In the production of lactide, byproduct streams containing vapor mixtures of lactide are often produced, and it is desirable to scrub the byproduct streams to reduce their content of volatile organic compounds and/or to recover the lactide as lactic acid for recycling back into the process. For example, the vapor mixture containing lactide may originate from the vacuum overhead system of the reactor system or from distillates in the purification by distillation of the crude lactide product stream obtained in the process. This crude lactide product stream is typically a complex multicomponent mixture containing lactide, water, lactic acid and oligomers.
Vapor mixtures containing lactide are also a common byproduct stream in the production of polylactic acid (PLA) polymers, as in the ring-opening polymerization of lactide. For example, it is desirable to remove residual lactide monomer and other ‘lows’ from the PLA in order improve the product polymer properties such as its melt, molecular weight and color stability and/or mechanical properties.
Residual lactide and other ‘lows’ may be removed from the PLA by conventional devolatilization methods such as those based on applying low pressure and/or inert gas flow together with temperatures sufficiently high to cause the removal of lactide and other lows from the PLA by distillation. Equipment appropriate for the devolatilization of PLA includes flash evaporators, falling strand devolatizers, thin film evaporators, high volume melt kneaders, and vented single-screw and twin-screw extruders. Vapor mixtures containing lactide may originate from any of these types of equipment and their devolatization processes. The devolatilization process may be integrated into the production of the PLA by having an in-line devolatilzation system after the final reactor in the polymerization and/or the devolatilization may be done in a post-polymerization process. As in the case of the production of lactide, it is often desirable to reduce the content of volatile organic compounds prior to discharge of the byproduct streams and/or to recover the lactide and volatile oligomers for recycling as raw materials into the process to produce lactide monomer and/or to produce PLA.
Processes for the removal of lactide from vapor streams are known. For example EP2030667, which is hereby incorporated by reference, discloses a method and a device for the condensation and washing of process vapors occurring during the production of polylactide. It is disclosed that the process vapors containing lactide may be condensed and/or washed by bringing them into contact with a stream of a condensation and washing liquid containing an aqueous solution of lactic acid such that the lactide dissolves in the liquid. It is disclosed that it is preferred to conduct the condensation and washing liquid in a circulation, particularly in the case of a continuous plant for the production of polylactide.
However in the method of EP2030667, the lactide-containing vapors which are condensed lead to an increase in the concentration of the lactide in the condensation and washing liquid. As a result, the solubility limit of lactide is exceeded, and solids precipitate in the circulation liquid causing blockages in the circulation, particularly in the packed bed or in the mass transfer element of the condensation and/or washing column. In addition, the lactide reacts with the water contained in the liquid by ring-opening to form lactoyllactic acid. As a result of the lactoyllactic acid formation, the viscosity of the liquid increases and the distribution over the bed or packing is hindered, and the condensation and washing effect is reduced. EP2030667 thus discloses that it is necessary to supply a mixture of water and lactic acid continuously or in portions to the circulated condensation and washing liquid so that the solubility limit of the lactide in the circulation is not reached and the viscosity of the liquid mixture does not rise. In order to avoiding flooding of the system, it is then necessary to extract a partial flow of the liquid from the circuit, which corresponds to the sum of the rate of flow of the mixture of water and lactic acid and the rate of flow of the condensate into the circuit. Therefore this method requires complex and cumbersome methods in order to avoid blockages and to maintain the proper balance of flows.
A further disadvantage of the method of EP2030667 is that lactide has very poor solubility in water, lactic acid and their mixtures, and lactide also has a relatively slow rate of dissolution and hydrolysis in these liquids. Therefore large amounts of water and lactic acid must be added to avoid precipitation. Adding large amounts of these liquids is counterproductive in that the function of the condensation and washing method and apparatus should be to remove components coming from the process, such as those originating from the vapor mixture, and not to add further components to the process.
Slow rates of dissolution and hydrolysis may be overcome by increasing the temperature of the condensation and washing liquid in EP2030667. However increasing the temperature simultaneously increases then the vapor pressure of the liquid in the column. The column though is in fluid communication with the devolatilization vessel, which is the source of the vapor mixture containing lactide. Therefore increasing the temperature and thus pressure in the column will also increase the pressure in the devolatilization vessel and thus negatively impact the removal of lactide and other lows by distillation in that vessel.
Alternatively, the saponification of esters by bases is known, and a caustic solution such as a NaOH solution may be added to the column in order to catalyze the hydrolysis of the ester and convert it to its sodium salt. However, this method requires the controlled addition of additional substances to the process in their necessary amounts, and the use of caustic solutions is generally undesirable due to the corrosion and EHS problems associated with their use. Furthermore considerable heat is generated when caustic is mixed with water, and this may lead to uncontrolled and violent boiling and/or splattering in the process if the caustic solution becomes concentrated in one area, or if it is added too rapidly or to aqueous solution that is too cold or too hot.
In conclusion, it would be desirable to have a method and apparatus for removing esters such as lactides from vapor mixtures that would be easier to control than that of EP2030667, and that is able to prevent the precipitation of solids and increases in viscosity in the system without the need for adding large amounts of additional components or caustic solutions to the circulation and while allowing a low liquid temperature and thus pressure to be maintained in the column.